Facile preparation of silicon hollow spheres and their use in electrochemical capacitive energy storage

Article abstract of DOI:10.1039/c2cc17083c

Nanostructured silicon hollow spheres with a thin shell have been synthesized by magnesium reduction of silica spheres, which possess a high BET surface area and are electrochemically active in capacitive energy storage with a maximum specific capacitance of 193 F g^-1 in the neutral Na ₂SO₄ aqueous solution. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17083c

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of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, China
Facile preparation of silicon hollow spheres and their use in
electrochemical capacitive energy storage
Nanostructured silicon hollow spheres with a thin shell have been
synthesized by magnesium reduction of silica spheres, which
possess a high BET surface area and are electrochemically active in
capacitive energy storage with a maximum specific capacitance of
193 F g-1 in neutral Na2SO4 aqueous solution.
See Hong-Bin Du et al.,
Chem. Commun., 2012, 48, 4950.
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COMMUNICATION
Facile preparation of silicon hollow spheres and their use in
electrochemical capacitive energy storagew
Mei-Pin Liu, Cheng-Hui Li, Hong-Bin Du* and Xiao-Zeng You
Received 15th November 2011, Accepted 11th January 2012
DOI: 10.1039/c2cc17083c
Nanostructured silicon hollow spheres with a thin shell have been
synthesized by magnesium reduction of silica spheres, which
possess a high BET surface area and are electrochemically active
in capacitive energy storage with a maximum specific capacitance
of 193 F gÀ1 in the neutral Na2SO4 aqueous solution.
and polymer-templated silica thin films, respectively. This
communication presents the preparation of crystalline silicon
hollow spheres by magnesium reduction of nanostructured
silica spheres and their use in capacitive energy storage.
Nanostructured silica spheres were fabricated by hydrolyzing
tetraethylorthosilicate (TEOS) with cetyltrimethylammonium
bromide (CTAB) as a template according to the modified literature
procedure.w12 The obtained silica spheres were converted into
MgO/Si replicas with gaseous Mg as the reducing agent as shown
in eqn (1):
Silicon, the second most abundant element in Earth’s crust, is
one of the most industrially important materials because of its
unique semiconducting properties.1 Studies on this and related
semiconductor materials have attracted much attention due to
their wide applications in transistors, optoelectronics, solar
energy conversions, biologic sensors, photo detectors, etc.2
Nanostructured silicon materials such as nanoparticles, nanowires,
nanotubes and porous silicon have shown unique chemical, optical,
electronic, and mechanical properties distinct from those of the
corresponding bulk materials, which gives rise to their potential use
in the fields of luminescence, catalysis, energy storage, and
biomedical sciences.3 Among these the use of nanostructured
silicon in high-capacity energy storage lithium batteries has
recently attracted tremendous interest.4
2Mg(g) + SiO2(s) - 2MgO(s) + Si(s)
(1)
The resulting brown material was then washed with dilute HCl
and HF to remove MgO and unreacted SiO2 in the inner core
of the spheres, leading to dark-brown crystalline silicon hollow
spheres.
As shown in Fig. 1a (also Fig. S1 in ESIw), the silica
precursors are spherical in shape and well-dispersed with a
diameter of ca. 400 nm. N2 sorption measurements showed that
the precursors are highly porous with a Brunauer–Emmett–
Teller (BET) surface area of 928 m2 gÀ1 and an average Barrett–
Joyner–Halenda (BJH) pore size of 3.2 nm (Fig. S2 in ESIw).
Upon heating with Mg, the spheres became lumpy and coated
with small particles (Fig. 1b), which are crystallites of MgO. The
treatments with dilute HCl and HF selectively dissolved MgO and
unreacted SiO2, respectively, to yield silicon spheres. The resulting
silicon-based products retained the spherical morphology and are
hollow in nature as evidenced from some broken spheres (Fig. 1c).
The TEM image in Fig. 1d further confirms the hollow
structure of the silicon spheres. The thickness of the spherical
shell was estimated as ca. 30 nm. The energy dispersive X-ray
pattern in Fig. 1e shows that the spheres after treatments with
HCl and HF consist of almost pure silicon. The presence of trace
oxygen is likely due to the contamination of the carbon tape used
for binding samples, although there were also possibilities that a
slight surface oxidation in air occurred during transferring the
samples into the chamber. As depicted in Fig. 1f, the powder X-ray
diffraction (XRD) pattern shows that the spheres are pure crystal-
line silicon, the peaks of which can be indexed to a cubic phase
(JCPDS 27-1402) with a = 5.43(1) A. Convolution-based profile
fitting of the XRD pattern by using the TOPAS package13 yielded
an average silicon crystallite size of 32 nm (Rwp = 11.4%), which is
in accordance with the wall thickness estimated from TEM images.
Hollow structured materials have been of particular interest
because of their low density, increased surface area and high
porosity.5 Various synthetic strategies have been developed to
prepare this type of nanostructured inorganic materials, such as
template-directed synthesis,6 self-curing,7 solvent extraction,8 and
so on. However, due to the high crystallization energy barrier of
silicon, these strategies can hardly be used to synthesize hollow
structured silicon. Nonetheless, hollow structured silicon has
been made by chemical vapor deposition (CVD) of silicon
precursors with hard templates,9 and thermal decomposition of
silicon precursors.3 These methods are usually of high cost and
not easy to scale up. Herein, we report a facile two-step synthesis
of crystalline silicon hollow spheres by using a magnesiothermic
reduction process. This method has been used to produce the
three-dimensional microporous nanocrystalline silicon replicas10
and ordered mesoporous silicon films11 by magnesium reductions
of nanostructured silica microshells of diatoms (unicellular algae)
State Key Laboratory of Coordination Chemistry and Nanjing
National Laboratory of Microstructures, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, 210093, China.
E-mail: hbdu@nju.edu.cn
w Electronic supplementary information (ESI) available: Synthesis, char-
acterization methods and CV curves. See DOI: 10.1039/c2cc17083c
c
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Fig. 2 Nitrogen adsorption (solid symbols) and desorption (empty
symbols) isotherms of (a) commercial Si powder, (b) silica precursors,
and (c) silicon hollow spheres at 77 K. Inset shows the BJH pore
distribution curve of silicon hollow spheres.
Fig. 1 SEM images of (a) silica precursors, (b) untreated silicon
products, and (c) silicon spheres after treatments with HCl and HF;
(d) TEM image of treated silicon spheres; and (e) energy-dispersive
X-ray spectra and (f) XRD pattern of silicon hollow spheres.
Fig. 3 CV curves of a silicon hollow sphere electrode in a 0.5 mol LÀ1
Na2SO4 solution at a scanning rate of (a) 100, (b) 50, (c) 20, (d) 10, and
(e) 5 mV sÀ1, respectively.
As shown in Fig. 2, nitrogen adsorption and desorption
isotherms of silicon hollow spheres are of type II with an H3
hysteresis, indicating the presence of micropores and mesopores
formed by agglomerates of silicon nanoparticles within spherical
shells. The calculated BET surface area of Si hollow spheres is
235 m2 gÀ1 and the Langmuir surface area is 322 m2 gÀ1. In
comparison, the commercial silicon with a particle size of 40 mm
shown in Fig. 3. The CV curves of the silicon hollow sphere
electrode exhibited a widely opened loop between 0.4–0.8 V,
implying good capacitive energy storage ability. In comparison,
commercial silicon (Sigma-Aldrich, À325 mesh) showed a very
narrow loop, even narrower than that of the conducting additive
acetylene blacks under the same test conditions (Fig. S3 in ESIw).
The specific capacitances of the silicon hollow sphere electrode
from the CVs were calculated to be 193, 152, 116, 83, 62 F gÀ1 at
a scanning rate of 5, 10, 20, 50, 100 mV sÀ1, respectively. The
energy storage capacities are comparable to those of some best
carbon capacitors.15 Considering the higher density of the silicon
materials than those of carbons, the silicon based capacitors have
higher volumetric energy density, which would make the silicon
capacitors potential candidates for applications.
has a BET and Langmuir surface area of 3 and 4 m2 gÀ1
,
respectively. The average BJH pore diameter of silicon hollow
spheres based on desorption curves is about 32 nm, which is
much larger than those of the silica precursors (3.2 nm) and
indicates that the mesopores in silicon hollow spheres mainly
resulted from agglomerates of silicon nanoparticles.
Porous silicon has been studied for use as a supercapacitor
for energy storage.14 However, it showed a relatively low
capacitance in comparison with carbon. Because in super-
capacitors charges are stored on the surface, the present silicon
hollow spheres with high surface areas are attractive for use in
supercapacitors. The electrochemical capacitive properties of silicon
hollow spheres were thus investigated by cyclic voltammetry (CV)
and chronopotentiometry with 0.5 mol LÀ1 aqueous Na2SO4 as
the electrolyte. It is interesting that the silicon hollow spheres
demonstrated excellent capacitive energy storage properties, as
Close examinations of the CV curves of the silicon hollow
sphere electrode revealed that the loops are not rectangular as
carbon capacitors usually exhibit. There exists a couple of
redox peaks at 0.45 and 0.65 V at a scanning rate of 5 mV sÀ1
,
which indicates that the capacity of the silicon hollow sphere
electrode probably arises from pseudocapacitive capacitance
based on the redox mechanism. Chronopotentiograms of the
c
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Chem. Commun., 2012, 48, 4950–4952 4951

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Na2SO4 aqueous solution, comparable to the carbon capacitors.
These studies show that controlling the size and shape of silicon
nanocrystallites would provide opportunities and possibilities
of tailoring properties of materials.
We are grateful for financial support from the National
Basic Research Program (2011CB808704), the National
Natural Science Foundation of China (21021062 and 21071075),
the Fundamental Research Funds for the Central Universities
(1114020502) and the Academician Work Station of Changzhou
Trina Solar Energy Co., Ltd.
Notes and references
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Fig. 4 Cyclability of a supercapacitor based on the silicon hollow
sphere electrode in a 0.5 mol LÀ1 Na2SO4 with a current density of
1 A gÀ1. Inset shows the charge–discharge curves for the first 2000 s.
silicon hollow sphere electrode in the potential range of
0.3–0.8 V are shown in Fig. 4. The capacitances calculated
from the chronopotentiograms are 145, 156, 166 F gÀ1 at a
current density of 1.25, 0.75, and 0.5 A gÀ1, respectively. The
charge–discharge curves deviate from ideal linear line due to
the redox reactions as evidenced by the cyclic voltammetric
study (Fig. 3). This indicates the existence of the double-layer
capacitance created by the charge separation at the electrode/
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As for an energy storage device, the cycle life is an important
parameter to evaluate its performance. Therefore, we per-
formed the charge/discharge tests to examine the cycle life of
the silicon hollow sphere electrode as a capacitor. As shown in
Fig. 4, the electrode was tested at a current density of 1 A gÀ1
.
A 10% loss of capacitance was observed during the first 100 cycles.
After 300 cycles, the capacitance retained ca. 61% of the starting
value, while ca. 27% of the capacitance remained after 600 cycles.
The degeneration of the capacitance after charge–discharge
processes resembles those of pseudocapacitors made of metal
oxides and organic polymers, and again implies the pseudo-
capacitive behaviours of the silicon hollow sphere electrode.
We postulate that the pseudocapacitance of the silicon hollow
sphere electrode resulted from the redox reactions of surface
silicon or silicon hydride as shown in eqn (2) and (3), which
consumed the active composition of the electrode, leading to
the degeneration of the capacitance. The silicon hydrides were
formed when the silicon hollow spheres were treated with HF.
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ꢀSi + H2O À eÀ - ꢀSiOH + H+
(2)
(3)
ꢀSiH + H2O À 2eÀ - ꢀSiOH + 2H+
In conclusions, nanostructured silicon hollow spheres with a thin
shell have been synthesized by magnesium reduction of silica
spheres. The silicon hollow spheres exhibit a high BET surface
area and are electrochemically active in capacitive energy storage
with a maximum specific capacitance of 193 F gÀ1 in the neutral
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c
4952 Chem. Commun., 2012, 48, 4950–4952
This journal is The Royal Society of Chemistry 2012